1. Field of the Invention
The present invention relates to a light wave-length converting apparatus of fiber type.
2. Description of Background Information
There has been known an optical pickup in which a wave-length of a laser light beam which is generated from a laser light source is converted into a half wave-length by using a light wave-length converting apparatus, thereby enabling information to be written onto and to be read out from an optical disk at a higher density (reference is directed to Japanese Patent Application Provisional Publicaton No. 61-50122).
A fiber type SHG (Second Harmonics Generator) using a secondary nonlinear optical effect has been known as a light wave-length converting apparatus. The fiber type SHG uses the phase matching of Cerenkov irradiation system. According to such a system, the second harmonic (hereinafter, abbreviated to an SH wave) is generated, to which phase matching is attained almost automatically.
In the fiber type SHG using the Cerenkov phase matching, the emerging SH wave has a ring-like intensity distribution and its equiphase surface is conical. To focus such an SH wave to a diffraction limit, as shown in FIG. 1, and optical system is generally used which comprises a combination of: an axicon lens 3 for converting an SH wave which was emitted from the primary light source 1 and wave-length converted by a fiber type SHG 2 into a parallel light; and a condenser lens 4 for focusing the parallel light derived from the axicon lens 3. On the other hand, in the case of using such an optical system as the light source of the optical pickup mentioned above, an optical system as shown in FIG. 2 is conceivable.
In FIG. 2, the parallel light from the axicon lens 3 passes through a beam splitter 5 and is transmitted through a quarter wave-length (.lambda./4) plate 6. Thereafter, the light is focused onto an information recording surface of an optical disk 8 by an objective lens 7 (corresponding to the condenser lens 4 in FIG. 1). The reflected light from the information recording surface passes through the objective lens 7 and the quarter wave-length plate 6. After that, the light is reflected by the beam splitter 5. The reflected light then passes through a condenser lens 9 and a cylindrical lens 10 and enters into the photo sensitive surface of a photodetector 11.
As mentioned above, in the case of the optical pickup using the fiber type SHG including the combination of the axicon lens 3 and the objective lens 7 as a light source, since the light beam has a ring-like intensity distribution, the diameter of the light beam must be adjusted so as to sufficiently satisfy conditions determined a numerical aperture (NA) of the objective lens 7. Further, if the objective lens 7 is vibrated in the direction perpendicular to the optical axis by a tracking servo operation, the ring-like light beam would be interrupted by the objective lens 7, so that a drawback will result such that the intensity distribution becomes uneven and a focused light spot is disturbed.
On the other hand, as a focused pattern which is obtained by focusing the light having the ring-like intensity distribution, a side lobe light amount increases as compared with the pattern of a light having a circular intensity distribution. Therefore, what is called a crosstalk, that is the leakage of information of adjacent pits on the information recording surface, increases and a central lobe light amount decreases, so that an actual light amount which can be used to read pit information decreases. Such a situation causes a large problem in the case where an efficiency of the SHG itself is relatively low.
The above drawbacks are originated from the fact that the light of the light source has a ring-like intensity distribution.